Delta-sigma digital to analog converters (DACs) are known in the art. A delta-sigma DAC includes an interpolation filter, sample and hold, and delta-sigma modulator. These parts form the digital section, which produces a one bit digital output or sometimes a multi bit output. A DAC also includes an analog part with a digital-to analog converter and an analog filter.
Noise reduction in DAC systems is important in improving DAC performance. When a zero DC input is applied to a delta-sigma modulator, the output spectrum often has a strong tone near NxF.sub.s /2, where N is the oversampling ratio and F.sub.s is the sampling frequency of the DAC input data. If the DC level is increased or decreased, the tone will move to a lower frequency either directly or through aliasing. This strong tone is called the idle tone. The position of this undesirable idle tone will often modulate with noise on the DAC's reference voltage. This position of modulation may produce spurious tones within the signal passband due to noise on the reference at frequencies near NxF.sub.s /2. Noise at NxF.sub.s /2 frequency is easily coupled onto the reference by clocks or digital switching activity. So, a small DC input will cause the idle tone near NxF.sub.s /2 to modulate with noise at NxF.sub.s /2 to produce a spurious tone near DC. This may detract from the high quality of audio that is desired.